Multifunctional Hard Yet Flexible Coatings Fabricated Using a Universal Step‐by‐Step Strategy

Abstract Hard yet flexible coatings with multi‐functionalities are useful for foldable displays and marine industries but rare. In this study, a highly cross‐linked multifunctional hybrid coating with ceramic‐like hardness and polymer‐like flexibility is reported. The coating is prepared via a step‐by‐step strategy, where two types of epoxy‐oligosiloxane nanoclusters are first synthesized by sol‐gel chemistry, and amine‐terminated curing agents are used to cross‐link them at room temperature. The coating is highly transparent (>92% transmittance), hard (6‐7H), and flexible (10 mm bending diameter) because of the unique combination of siloxane nanoclusters and polymer networks. Meanwhile, since the coating contains fouling‐resistant telomer and low‐surface‐tension liquid lubricant polydimethylsiloxane (PDMS), it exhibits excellent anti‐biofouling and self‐cleaning properties. The results indicate that the mechanical and antifouling properties of the coating can be easily tuned and prove that the step‐by‐step strategy is a promising and universal method. The novel coatings can meet the needs of applications in foldable displays, marine industries, and other fields.

Hydrogen chloride solution (HCl, 37 wt%), ethanol (EtOH), and methanol were from Sinopharm and used as received. Tetrahydrofuran (THF) and hexane were from Sinopharm and distilled by CaH 2 . Xylene from Guangzhou Chemical Reagent Factory was used as received. PDMS elastomer (Sylgard 184) was purchased from Dow Corning.
Fouling resistant epoxy-oligosiloxane nanocluster: Silane-terminated telomer (S-FP) was synthesized according to the literature. [1] KH560 (23.6 g), H 2 O (3.1 g), HCl solution (0.2 g), and ethanol (20 mL) were stirred at 60 °C for 4 h. Then S-FP (9.0 g), H 2 O (0.2 g), and HCl 3 solution (0.1 g) were added to the mixture, which was continued to be hydrolyzed for another 4 h. After being concentrated under vacuum to remove volatiles, the fouling resistant epoxyoligosiloxane nanocluster was obtained, which was designated as FP.

Preparation of the hybrid coatings
The hybrid coatings were prepared by using different curing agents to cross-link the HP and FP nanoclusters. Typically, HP2-FP2 was prepared by mixing HP (1.3 g), FP (1.5 g) and D400 (1.0 g) in ethyl acetate (4 mL), then the mixture was stirred at 25 °C for 10 min. After stirring, the reacted solution was dropped on different substrates and dried at 25 °C for 7 d to ensure complete curing. When using APT-PDMS to replace part of the curing agents, APT-PDMS should be mixed with nanoclusters first and stirred at 25 °C for 6 h, then D400 was added to prepare the coatings. Following the similar protocol, other hybrid coatings were prepared.

Characterization
Nuclear Magnetic Resonance Spectroscopy (NMR). 1 H NMR Spectrum and 29 Si NMR spectra were performed on a Bruker AV600 NMR Spectrometer using CDCl 3 as the solvent and tetramethylsilane as the internal standard.
Pencil Hardness Tests. The pencil hardness of coating was measured according to the ASTM D3363 (2020). [2] Pencils with hardness ranging from 6 B (The softest) to 9 H (The hardest) were used to scratch along the coating surface at an angle of 45°. The pencil hardness of coating was equivalent to that of the hardest pencil that did not leave a scratch on the coating surface.
Flexibility Tests. The flexibility test was conducted according to GB/T1731-93 [3] by a coating flexibility tester (Shanghai Jingge Technology Co., Ltd, Model QTX). All of the coatings were coated on tinplate and bent on shaft rods of different diameters. The flexibility was 4 determined by the minimum diameter of the shaft rod that did not make the coating crack after bending.
Transmittance Spectra Measurements. All samples were coated on cleaned glass substrate with a thickness of ~60 μm. The UV-Vis spectra were recorded by a UV-Vis spectrometer (Agilent, Model Cary 3500).
Adhesion Tests. The adhesion tests were performed by an automatic tester (PosiTest, Model AT-A) according to ASTM D4541-09. [4] All samples were coated on steel, epoxy fiberglass, glass, and PET film for adhesion tests. Before test, the coating surface was polished lightly by abrasive paper to make the aluminum studs adhere better. The rate of pull was set to 0. UV Resistance Tests. The UV resistance of the coatings was measured by a QUV accelerated weathering tester (Q-Panel Lab, Model QUV/spray) according to ASTM G154. [5] All the coatings coated on glass were fixed in the instrument and exposed to UV irradiation for 24 h.
The test intensity was 0.83 W/m 2 and the test temperature was 50 °C. The transmittance spectra, WCAs, and pencil hardness of the coatings were measured before and after exposure.
Tests were performed in triplicate.
Liquids Resistance Tests. The liquid resistance of the coatings was conducted referring to ISO 2812-1-2017. [6] Hexane, xylene, methanol, ethanol, 0.1 M HCl and 0.1 M NaOH aqueous solutions were used as the test liquids. All the coatings coated on glass plate were completely immersed in the test liquids for 24 h at 25 °C, and then placed in the oven at 60 °C for 4 h to remove the residual liquid. The WCAs and pencil hardness of the coatings were measured before and after immersion. Tests were performed in triplicate.
Nanoindentation Tests. A nanoindentor (Anton Paar, Model TTX-NHT3) equipped with Berkovich diamond tip was used for nanoindentation tests at 25 °C. Oliver-Pharr method was used to quantify the hardness (H IT ) and elastic modulus (E IT ). The loading and unloading rate were both set to 10.00 mN/min. All of the samples were coated at glass substrate with thickness of ~100 μm and their hardness (H IT ) and elastic modulus (E IT ) were calculated by computer from loading-displacement curve. [7] Scratch Resistant Tests. A 6 H pencil was used to scratch on the surface of bare PET substrate and hybrid coatings. After 20 cycles of scratching, the scratches on the surface of all samples were recorded.
Wear Tests. Abrasion tests were performed on an Abrasion Resistance Tester (ZL-7013B).
All of the samples were fixed on a horizontal board, then a test rod with a moving arm was set to compress a steel wool mat tightly on the sample. Abrasion was realized by the movement of arm and thus the steel wool mat can run back and forth on the coating surface at a rate of 20 r/min. After abrasion, the surface wear of the coatings was recorded using an optical microscope.
Bending Tests. All of the coatings were coated on PET substrate with a thickness of ~30 μm.
The coated film was bent to a U shape and then released. The bending and releasing processes were repeated 20 times.
Folding and Rolling up test. HP2-FP2 was coated on 5 short PET films (10 × 5 cm) and a long PET film (30 × 5 cm). The former was folded into letters "S", "C", "U" and "T" sequentially, and the latter was rolled up from both sides, and finally formed two reels. Tests were performed in triplicate. Anti-smudge test. The hybrid coatings were coated on the right half of different substrates (glass, tinplate, PET, wood, steel, and ceramic) while the left half of substrates was bare. An oily pen was used to write on the surface and the writings were observed. Then, napkin was used to wipe off the writings, the erasure of writings was recorded.
Self-cleaning test. Dust was covered on the surface of a coated glass. Then, water droplets were dropped on the coating surface to remove dust, the removal of dust was recorded.
Oil sliding test. Uncoated tinplate and coated tinplate was immersed in soybean oil for 12 h and lifted up, respectively. The time of all soybean oil slipped from the tinplate was recorded.
Surface lubricity test. Two wooden cubes were placed on the inclined coated and uncoated glass to slide off, respectively. The uncoated glass was inclined at 30° and the coated glass was inclined at 15°. The sliding time of cubes from glass top to bottom was recorded.
Pseudobarnacle removal tests. Following the standard of ASTM D5610 (2011), [8] 5 cylindrical aluminum studs (10 mm diameter) were glued on the coating surface using epoxy adhesive (Araldite). A shear force parallel to the surface was applied to the studs using a force gauge (SUNDOO, Model SH-500) to measure the removing strength.
Statistical analysis. Data are expressed as the mean ± standard deviation (SD). Statistical analysis was performed using one-way ANOVA testing. Significance was defined as p < 0.05.